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Anton Leeuwenhoek invented the microscope in
the late 1600’s, which first showed that all living
things are composed of cells. Also, he was the
first to see microorganisms.
In 1655, the English scientist Robert Hooke coined
the term “cellulae” for the small box-like structures
he saw while examining a thin slice of cork under a
In 1838 – 1839, German scientists Schleiden and
Schwann, proposed the first 2 principles of the cell
• All organisms are composed of one or more
• Cells are the smallest living units of all living
15 years later, the German physician Rudolf
Virchow proposed the third principle:
• Cells arise only by division of a previously
Basic Cell Structure
All cells have the following basic structure:
A thin, flexible plasma membrane surrounds the
entire cell that regulates the passage of materials
between the cell and its surrounding
The interior is filled with a semi-fluid material called
At some point, all cells contain DNA, the heritable
material that directs the cell’s activities
Also inside some cells are specialized structures
The cell is the lowest level of structure that is
capable of performing all the activities of life.
• Regulate its internal environment.
• Take in and use energy.
• Respond to its local environment.
• Develop and maintain its complex
• Divide to form new cells.
Two major kinds of cells - prokaryotic cells and
eukaryotic cells - can be distinguished by their
Eukaryotic cells - Contain membrane-enclosed
organelles, including a DNA-containing nucleus
Prokaryotic cells - Lack such organelles
The cells of the microorganisms called bacteria
and archaea are prokaryotic.
All other forms of life have the more complex
Prokar yotic cell
Some organisms consist of a single cells, others are
multicellular aggregates of specialized cells.
Multicellular Organisms exhibit three major structural
levels above the cell:
• Similar cells are grouped into tissues
• Several tissues coordinate to form organs
• Several organs form an organ system.
Cell Size Limit
Most cells are relatively small because as size increases,
volume increases much more rapidly than surface area.
• longer diffusion time
• limit to the volume of cytoplasm that can be
effectively controlled by genes.
Cell Size Limit
A cell must exchange materials with its
environment. Cell volume determines the
amount of materials that must be
exchanged, while surface area limits how
fast exchange can occur. In other words, as
cells get larger the need for materials
increases faster than the ability to absorb
• Cytoplasm is surrounded by plasma membrane and
encased in a rigid cell wall composed of peptidoglycan.
• No distinct interior compartments
• Some use flagellum for locomotion, threadlike structures
protruding from cell surface
Characterized by compartmentalization by
an endomembrane system, and the
presence of membrane-bound organelles.
• central vacuole
• cell walls
Fluid Mosaic Model
Repository for genetic material
Chromatin: DNA and proteins
Nucleolus: Chromatin and ribosomal
subunits - region of intensive ribosomal RNA
Nuclear envelope: Surface of nucleus bound
by two phospholipid bilayer membranes Double membrane with pores
Nucleoplasm: semifluid medium inside the
Protein synthesis occurs at tiny
organelles called ribosomes.
Ribosomes are composed of a large
subunit and a small subunit.
Ribosomes can be found alone in the
cytoplasm, in groups called
polyribosomes, or attached to the
Ribosomes are RNA-protein complexes
composed of two subunits that join and
attach to messenger RNA.
• site of protein synthesis
• assembled in nucleoli
Compartmentalizes cell, channeling
passage of molecules through cell’s interior.
• Endoplasmic reticulum
• Rough ER - studded with ribosomes
• Smooth ER - few ribosomes
Rough ER is especially abundant in cells that secrete proteins.
As a polypeptide is synthesized on a ribosome attached to rough ER, it is threaded into the
cisternal space through a pore formed by a protein complex in the ER membrane.
As it enters the cisternal space, the new protein folds into its native conformation.
Most secretory polypeptides are glycoproteins, proteins to which a carbohydrate is
Secretory proteins are packaged in transport vesicles that carry them to their next stage.
Rough ER is also a membrane factory.
Membrane-bound proteins are synthesized directly into the membrane.
Enzymes in the rough ER also synthesize phospholipids from precursors in the cytosol.
As the ER membrane expands, membrane can be transferred as transport vesicles to other
components of the endomembrane system.
The smooth ER is rich in enzymes and plays a role in a variety of metabolic
Enzymes of smooth ER synthesize lipids, including oils, phospholipids, and steroids.
These include the sex hormones of vertebrates and adrenal steroids.
In the smooth ER of the liver, enzymes help detoxify poisons and drugs such as
alcohol and barbiturates.
Smooth ER stores calcium ions.
Muscle cells have a specialized smooth ER that pumps calcium ions from
the cytosol and stores them in its cisternal space.
When a nerve impulse stimulates a muscle cell, calcium ions rush from
the ER into the cytosol, triggering contraction.
The Golgi apparatus
The Golgi apparatus is the shipping and receiving center for cell
• Many transport vesicles from the ER travel to the Golgi apparatus
for modification of their contents.
• The Golgi is a center of manufacturing, warehousing, sorting, and
• The Golgi apparatus consists of flattened membranous sacs—
cisternae—looking like a stack of pita bread.
• The Golgi sorts and packages materials into transport vesicles.
Functions Of The Golgi Apparatus
(“receiving” side of
1 Vesicles move
2 Vesicles coalesce to
6 Vesicles also
form new cis Golgi cisternae
from ER to Golgi
proteins back to ER
move in a cisto-trans
5 Vesicles transport specific
proteins backward to newer
0.1 0 µm
4 Vesicles form and
leave Golgi, carrying
specific proteins to
other locations or to
the plasma membrane for secretion
(“shipping” side of
TEM of Golgi apparatus
Membrane Bound Organelles
Lysosomes – vesicle
enzymes that break down
Vacuoles – food storage
and water regulation
Peroxisomes - contain
enzymes that catalyze the
removal of electrons and
(a) Phagocytosis: lysosome digesting food
• bounded by exterior and interior
• interior partitioned by cristae
• have enclosed internal compartments of
stacked grana, containing thylakoids
• found in photosynthetic organisms
Both organelles house energy in the form of
Mitochondria are found in plant and animal cells.
Sites of cellular respiration, ATP synthesis
Bound by a double membrane surrounding fluid-filled matrix.
The inner membranes of mitochondria are cristae
The matrix contains enzymes that break down carbohydrates and
the cristae house protein complexes that produce ATP
A chloroplast is bounded by two membranes enclosing
a fluid-filled stroma that contains enzymes.
Membranes inside the stroma are organized into
thylakoids that house chlorophyll.
Chlorophyll absorbs solar energy and carbohydrates
are made in the stroma.
The eukaryotic cytoskeleton is a network of filaments and
tubules that extends from the nucleus to the plasma
membrane that support cell shape and anchor organelles.
• Actin filaments - cell movement
• Microtubules - centrioles
• Intermediate filaments
Centrioles are short
cylinders with a 9 + 0
pattern of microtubule
Centrioles may be
involved in microtubule
disassembly during cell
division and in the
organization of cilia and
Cilia and Flagella
Contain specialized arrangements of microtubules
Are locomotor appendages of some cells
Cilia and flagella share a common ultrastructure
Cross section of basal body
Cilia and Flagella
Cilia (small and numerous) and flagella (large and single) have a
9 + 2 pattern of microtubules and are involved in cell movement.
Cilia and flagella move when the microtubule doublets slide past
Each cilium and flagellum has a basal body at its base.
Cilia and Flagella
(a) Motion of flagella. A flagellum
usually undulates, its snakelike
motion driving a cell in the same
direction as the axis of the
flagellum. Propulsion of a human
sperm cell is an example of
Direction of swimming
(b) Motion of cilia. Cilia have a backand-forth motion that moves the
cell in a direction perpendicular
to the axis of the cilium. A dense
nap of cilia, beating at a rate of
about 40 to 60 strokes a second,
covers this Colpidium, a
freshwater protozoan (SEM).
Long-lasting or permanent connections between
adjacent cells, 3 types of cell junctions:
Tight junctions prevent
fluid from moving
across a layer of cells
At tight junctions, the membranes of
neighboring cells are very tightly pressed
against each other, bound together by
specific proteins (purple). Forming continuous seals around the cells, tight junctions
prevent leakage of extracellular fluid across
A layer of epithelial cells.
Desmosomes (also called anchoring
junctions) function like rivets, fastening cells
Together into strong sheets. Intermediate
Filaments made of sturdy keratin proteins
Anchor desmosomes in the cytoplasm.
between Plasma membranes
of adjacent cells
Gap junctions (also called communicating
junctions) provide cytoplasmic channels from
one cell to an adjacent cell. Gap junctions
consist of special membrane proteins that
surround a pore through which ions, sugars,
amino acids, and other small molecules may
pass. Gap junctions are necessary for communication between cells in many types of tissues,
including heart muscle and animal embryos.
Connect cells into sheets. Because these
junctions form a tight seal between cells, in
order to cross the sheet, substances must
pass through the cells, they cannot pass
between the cells.
Attach the cytoskeleton of a cell to the matrix
surrounding the cell, or to the cytoskeleton
of an adjacent cell.
Link the cytoplasms of 2 cells together,
permitting the controlled passage of small
molecules or ions between them.
Two adjacent connexons
form a gap junction
(1 – 5 μm)
(10 – 100 μm)
infoldings of the
but usually lack
Complex system of
divides cell into
Smaller and free
in the cytoplasm
Larger and may
be bound to ER
Many linear chromosomes,
each made of 1 DNA
molecule joined with
In an area of the
cytoplasm called the
Inside a membrane-bound